Method and apparatus for verifying a determined cardiac event in a medical device based on detected variation in hemodynamic status

ABSTRACT

A method and apparatus for verifying a determined cardiac event in a medical device based on detected variation in hemodynamic status that includes a plurality of sensors sensing cardiac signals, and a physiologic sensor sensing physiologic signals to generate a plurality of variation index samples corresponding to the sensed signals. A microprocessor detects a cardiac event in response to the sensed cardiac signals, computes a variation index trend associated with a predetermined number of variation index samples of the plurality of variation index samples, determines a rate of change of the computed variation index trend, and confirms the determined cardiac event in response to the computed variation index trend and in response to the determined rate of change.

RELATED APPLICATION

The present application claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 60/787,765, filed Mar. 31, 2006,entitled “METHOD AND APPARATUS FOR VERIFYING A DETERMINED CARDIAC EVENTIN A MEDICAL DEVICE BASED ON DETECTED VARIATION IN HEMODYNAMIC STATUS”,incorporated herein by reference in its entirety.

CROSS-REFERENCE TO RELATED APPLICATION

Cross-reference is hereby made to the commonly-assigned related U.S.patent application Ser. No. 11/380,855, entitled “METHOD AND APPARATUSFOR VERIFYING A DETERMINED CARDIAC EVENT IN A MEDICAL DEVICE BASED ONDETECTED VARIATION IN HEMODYNAMIC STATUS”, to Bhunia; and U.S. patentapplication Ser. No. 11/380,849, entitled “METHOD AND APPARATUS FORVERIFYING A DETERMINED CARDIAC EVENT IN A MEDICAL DEVICE BASED ONDETECTED VARIATION IN HEMODYNAMIC STATUS”, to Bhunia, both filedconcurrently herewith and incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates generally to medical devices, and moreparticularly to a method and apparatus for confirming detection of acardiac event based on the detection variations in hemodynamic statususing an optical sensor.

BACKGROUND OF THE INVENTION

Implantable medical devices (IMDs) for monitoring a physiologicalcondition or delivering a therapy typically rely on one or more sensorspositioned in a patient's blood vessel, heart chamber, or other portionof the body. Examples of such medical devices include heart monitors,pacemakers, implantable cardioverter-defibrillators (ICDs),myostimulators, nerve stimulators, drug delivery devices, subcutaneousdefibrillators, and other IMDs where such sensors are desirable.Implantable sensors used in conjunction with an IMD generally provide asignal related to a physiological condition from which a patientcondition or the need for a therapy can be assessed.

Measurement of blood oxygen saturation levels are of interest indetermining the metabolic state of the patient. Generally, a decrease inblood oxygen saturation is associated with an increase in physicalactivity or may reflect insufficient cardiac output or respiratoryactivity. Thus monitoring blood oxygen saturation allows an implantablemedical device to respond to a decrease in oxygen saturation, forexample by pacing the heart at a higher rate. An implantable oxygensensor for use with an implantable medical device is generally disclosedin commonly assigned U.S. Pat. No. 6,198,952 issued to Miesel, herebyincorporated herein by reference in its entirety. Cardiac pacemakersthat respond to changes in blood oxygen saturation as measured by anoptical sensor are generally disclosed in U.S. Pat. No. 4,202,339 issuedto Wirtzfeld and in U.S. Pat. No. 4,467,807 issued to Bornzin.

Practical applications for optical hemodynamic sensors, however, havebeen limited because such sensors are highly susceptible to motion; thatis, movement by the patient or of the sensor tends to introducesignificant noise onto an output signal of the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and features of the present invention will be appreciated as thesame becomes better understood by reference to the following detaileddescription of the embodiments of the invention when considered inconnection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an exemplary medical device in whichthe present invention may be usefully practiced;

FIG. 2 is a schematic diagram of an optical hemodynamic sensor accordingto an embodiment of the present invention;

FIG. 3 is a schematic diagram of electronic circuitry included in thedevice of FIG. 1 according to an embodiment of the present invention;

FIG. 4 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention;

FIG. 5 is a graphical representation of identification of an exemplaryO₂ variation index trend utilized in a method of delivering a therapy ina medical device according to an embodiment of the present invention;

FIG. 6 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention;

FIG. 7 is an exemplary O₂ variation index trend identified in a methodof delivering a therapy in a medical device according to an embodimentof the present invention;

FIG. 8 is an exemplary graphical representation of an O₂ variation indextrend utilized in a method of delivering a therapy in a medical deviceaccording to an embodiment of the present invention;

FIG. 9 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention;

FIG. 10 is an exemplary graphical representation of an O₂ variationindex trend utilized in a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention;

FIG. 11 is an exemplary graphical representation of generation of acorrected O₂ variation index trend offset utilized in a method ofdelivering a therapy in a medical device according to an embodiment ofthe present invention;

FIG. 12 is an exemplary graphical representation of an O₂ variationindex trend utilized in a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention;

FIG. 13 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention;

FIG. 14 is an exemplary graphical representation of an O₂ variationindex trend utilized in a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention; and

FIG. 15 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of an exemplary medical device in whichthe present invention may be usefully practiced. As illustrated in FIG.1, the present invention may be utilized in an implantable medicaldevice 14 that includes a housing 15 containing circuitry for operatingdevice 14 that is subcutaneously implanted in a patient, outside theribcage of patient 12, anterior to the cardiac notch, for example.According to an embodiment of the present invention, housing 15 may beimplanted in the pectoral region of the patient 12. Further, device 14may include a subcutaneous sensing and cardioversion/defibrillationtherapy delivery lead 18 coupled to the device 14 that is tunneledsubcutaneously into a location adjacent to a portion of a latissimusdorsi muscle of patient 12. Specifically, lead 18 is tunneledsubcutaneously from the median implant pocket of device 14 laterally andposterially to the patient's back to a location opposite the heart suchthat the heart 16 is disposed between the device 14 and the distalelectrode coil 24 and distal sensing electrode 26 of lead 18.

It is understood that while the subcutaneous device 14 is shownpositioned through loose connective tissue between the skin and musclelayer of the patient, the term “subcutaneous device” is intended toinclude a device that can be positioned in the patient to be implantedusing any non-intravenous location of the patient, such as below themuscle layer or within the thoracic cavity, for example.

Further referring to FIG. 1, programmer 20 is shown in telemetriccommunication with SubQ ICD 14 by RF communication link 22.Communication link 22 may be any appropriate RF link such as Bluetooth,WiFi, MICS, or as described in U.S. Pat. No. 5,683,432 “AdaptivePerformance-Optimizing Communication System for Communicating with anImplantable Medical Device” to Goedeke, et al and incorporated herein byreference in its entirety.

Device 14 may be constructed of stainless steel, titanium or ceramic asdescribed in U.S. Pat. Nos. 4,180,078 “Lead Connector for a BodyImplantable Stimulator” to Anderson and 5,470,345 “Implantable MedicalDevice with Multi-layered Ceramic Enclosure” to Hassler, et al, bothincorporated herein by reference in their entireties. The electronicscircuitry of device 14 may be incorporated on a polyamide flex circuit,printed circuit board (PCB) or ceramic substrate with integratedcircuits packaged in leadless chip carriers and/or chip scale packaging(CSP).

Lead 18, which is inserted within a connector (not shown) positioned onhousing 15 to electrically coupled lead to the circuitry located inhousing 15, includes a distal defibrillation coil electrode 24, a distalsensing electrode 26, an insulated flexible lead body and a proximalconnector pin (not shown) for connection to housing 15 via theconnector. Distal sensing electrode 26 is sized appropriately to matchthe sensing impedance of one or more electrodes 28 that are positionedalong housing 15 to form a housing-based subcutaneous electrode arraywith electrodes 28 positioned to form orthogonal signal vectors.

Device 14 in is an exemplary graphical representation of an O₂ variationindex trend utilized in a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention includes anoptical sensor 17 positioned along the outer surface of housing 15,which is utilized to generate an O₂ variation index trend for use ingenerating a secondary confirmation of the detection of a cardiac eventby the primary detection algorithm, as described in detail below.Electrodes 28 and optical sensor 17 are welded into place on the outersurface of the housing 15 and are connected via wires (not shown) toelectronic circuitry (described herein below) located inside housing 15.Electrodes 28 may be constructed of flat plates, or alternatively,spiral electrodes as described in U.S. Pat. No. 6,512,940 “SubcutaneousSpiral Electrode for Sensing Electrical Signals of the Heart” to Brabec,et al and mounted in a non-conductive surround shroud as described inU.S. Pat. Nos. 6,522,915 “Surround Shroud Connector and ElectrodeHousings for a Subcutaneous Electrode Array and Leadless ECGs” toCeballos, et al and 6,622,046 “Subcutaneous SensingFeedthrough/Electrode Assembly” to Fraley, et al, all incorporatedherein by reference in their entireties.

The electronic circuitry employed in device 14 can take any of the knownforms that detect a tachyarrhythmia from the sensed ECG and providecardioversion/defibrillation shocks as well as post-shock pacing asneeded while the heart recovers. An exemplary simplified block diagramof such circuitry adapted to function employing the first and secondcardioversion-defibrillation electrodes as well as the ECG sensing andpacing electrodes described herein below is set forth in FIG. 3. It willbe understood that the simplified block diagram does not show all of theconventional components and circuitry of such devices including digitalclocks and clock lines, low voltage power supply and supply lines forpowering the circuits and providing pacing pulses or telemetry circuitsfor telemetry transmissions between the device 14 and externalprogrammer 20.

Optical hemodynamic sensor 17 is preferably a multiple waveformoximeter, such as a pulse oximeter or a mixed-venous oxygen sensor, forexample. Pulse oximeters are well known sensors commonly used withvarious medical devices, both implantable and external. For example,some applications of optical oximeters are disclosed in U.S. Pat. No.4,750,495; 5,176,137; 6,144,866; 6,198,952; or 6,944,488, each of whichis assigned to Medtronic, Inc.

Generally, optical oximeters include a light source for emitting lightthrough a blood perfused tissue of patient P and a light detector forgenerating a signal representative of an intensity of light transmittedthrough the blood perfused tissue to the light detector. In otherembodiments, the mixed-venous oxygen sensor may be placed in the bloodstream itself. The light passed through the tissue or bloodstream iscommonly selected to be of two or more wavelengths, and most commonly,the light is selected to fall in the red part of the visible lightspectrum and the infrared (IR) portion of the light spectrum. The lighttransmitted through the blood perfused tissue or bloodstream andreceived by the light detector is generally representative ofhemodynamic function.

FIG. 2 is a schematic diagram of an optical hemodynamic sensor accordingto an embodiment of the present invention. As illustrated in FIG. 2optical sensor 17 includes a red light emitting diode (LED) 21, aninfrared (IR) LED 23, a photodiode 30, and an optical barrier 32, all ofwhich are positioned within a sensor housing 34 having a lens 36. In theembodiment shown in FIG. 2, LEDs 21 and 23 and photodiode 30 are eachmounted on a substrate 37, or a bottom surface of housing 34. Asindicated by arrows 38, red and IR LEDs 21 and 23 are configured to emitlight through lens 36 of housing 34, while, as indicated by arrows 39,photodiode 30 is configured to detect light received through lens 36.Optical barrier 32 is positioned to block direct transmission of lightfrom LEDs 21 and 23 to photodiode 30.

Optical hemodynamic sensor 17 preferably is subcutaneously orsubmuscularly implanted within patient P such that lens 36 is orientedtoward a blood perfused tissue of patient P.

Red LED 21 preferably emits light in the red portion of the visiblelight spectrum, while IR LED 23 preferably emits IR light in the IRportion of the light spectrum. In alternate embodiments, opticalhemodynamic sensor 17 may include any two or more light sources forproducing at least two different wavelengths of light. Photodiode 30preferably receives light transmitted by LEDs 21 and 23, with anintensity of the signal received by photodiode 30 being indicative ofhemodynamic function. For instance, oxygen saturation of the blood canbe derived from an output of photodiode 30, as will be described below,and used to provide a secondary confirmation of a detected event by thedevice 14 according to the present invention.

FIG. 3 is a schematic diagram of electronic circuitry included in thedevice of FIG. 1 according to an embodiment of the present invention. Asillustrated in FIG. 3, device 14 includes both a low voltage battery 153and a high voltage battery 112, for example, positioned within thehermetically sealed housing 15 of the device 14. Low voltage battery 153is coupled to a power supply (not shown) that supplies power to thedevice circuitry and the pacing output capacitors to supply pacingenergy in a manner well known in the art. The low voltage battery 153can include one or more conventional LiCF_(x), LiMnO₂ or LiI₂ cells,while the high voltage battery 112 can include one or more conventionalLiSVO or LiMnO₂ cells. It is understood that although the exemplaryembodiment of FIG. 3 includes both low and high power therapy, thepresent invention may be employed in a device that provides only onetherapy, such as a high power defibrillation therapy, for example.

Device 14 functions are controlled by means of software, firmware andhardware that cooperatively monitor the ECG, determine when acardioversion-defibrillation shock or pacing is necessary, and deliverprescribed cardioversion-defibrillation and pacing therapies. FIG. 3incorporates circuitry set forth in commonly assigned U.S. Pat. Nos.5,163,427 “Apparatus for Delivering Single and Multiple Cardioversionand Defibrillation Pulses” to Keimel and 5,188,105 “Apparatus and Methodfor Treating a Tachyarrhythmia” to Keimel for selectively deliveringsingle phase, simultaneous biphasic and sequential biphasiccardioversion-defibrillation shocks, incorporated herein by reference intheir entireties

In FIG. 3, sense amp 190 in conjunction with pacer/device timing circuit178 processes the far field ECG sense signal that is developed across aparticular ECG sense vector defined by a selected pair of thesubcutaneous electrodes 28 or, optionally, a virtual signal if selected.The selection of the sensing electrode pair is made through the switchmatrix/MUX 191 in a manner to provide the most reliable sensing of theEGM signal of interest, which would be the R wave for patients who arebelieved to be at risk of ventricular fibrillation leading to suddendeath. The far field ECG signals are passed through the switchmatrix/MUX 191 to the input of the sense amplifier 190 that, inconjunction with pacer/device timing circuit 178, evaluates the sensedEGM. Bradycardia, or asystole, is typically determined by an escapeinterval timer within the pacer timing circuit 178 and/or the controlcircuit 144. Pace Trigger signals are applied to the pacing pulsegenerator 192 generating pacing stimulation when the interval betweensuccessive R-waves exceeds the escape interval. Bradycardia pacing isoften temporarily provided to maintain cardiac output after delivery ofa cardioversion-defibrillation shock that may cause the heart to slowlybeat as it recovers back to normal function. Sensing subcutaneous farfield signals in the presence of noise may be aided by the use ofappropriate denial and extensible accommodation periods as described inU.S. Pat. No. 6,236,882 “Noise Rejection for Monitoring ECGs” to Lee, etal and incorporated herein by reference in its' entirety.

Detection of a malignant tachyarrhythmia is determined in the controlcircuit 144, for example, as a function of the intervals between R-wavesense event signals that are output from the pacer/device timing 178 andsense amplifier circuit 190 to the timing and control circuit 144.

Supplemental sensors such as tissue color, tissue oxygenation,respiration, patient activity and the like may be used to contribute tothe decision to apply or withhold a defibrillation therapy as describedgenerally in U.S. Pat. No. 5,464,434 “Medical Interventional DeviceResponsive to Sudden Hemodynamic Change” to Alt and incorporated hereinby reference in its entirety. In particular, the present inventionincludes optical sensor 17 to provide a secondary confirmation of adetected tachyarrhythmia event detected by the device 14 by determiningwhether the heart is hemodynamically unstable in response to atachycardia event being identified by the device 15 in response toR-wave sense intervals determined in the primary detection algorithm,described below in detail. Sensor processing unit 194 provides sensordata to microprocessor 142 via data bus 146. In addition to opticalsensor 17, an activity sensor may also be utilized so that patientactivity and/or posture may also be determined by the apparatus andmethod as described in U.S. Pat. No. 5,593,431 “Medical ServiceEmploying Multiple DC Accelerometers for Patient Activity and PostureSensing and Method” to Sheldon and incorporated herein by reference inits entirety. Similarly, patient respiration may be determined by theapparatus and method as described in U.S. Pat. No. 4,567,892“Implantable Cardiac Pacemaker” to Plicchi, et al and incorporatedherein by reference in its entirety. As mentioned above, according tothe present invention, optical sensor 17 may be located on the housing15 of device 14, or may be located on the lead 18 to enable the sensingof contacting or near-contacting tissue oxygenation.

Certain steps in the performance of the detection algorithm criteria arecooperatively performed in microcomputer 142, including microprocessor,RAM and ROM, associated circuitry, and stored detection criteria thatmay be programmed into RAM via a telemetry interface (not shown)conventional in the art. Data and commands are exchanged betweenmicrocomputer 142 and timing and control circuit 144, pacertiming/amplifier circuit 178, and high voltage output circuit 140 via abi-directional data/control bus 146. The pacer timing/amplifier circuit178 and the control circuit 144 are clocked at a slow clock rate. Themicrocomputer 142 is normally asleep, but is awakened and operated by afast clock by interrupts developed by each R-wave sense event, onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pacer/device timing circuitry 178.

The algorithms and functions of the microcomputer 142 and controlcircuit 144 employed and performed in detection of tachyarrhythmias areset forth, for example, in commonly assigned U.S. Pat. Nos. 5,354,316“Method and Apparatus for Detection and Treatment of Tachycardia andFibrillation” to Keimel; 5,545,186 “Prioritized Rule Based Method andApparatus for Diagnosis and Treatment of Arrhythmias” to Olson, et al,5,855,593 “Prioritized Rule Based Method and Apparatus for Diagnosis andTreatment of Arrhythmias” to Olson, et al and 5,193,535 “Method andApparatus for Discrimination of Ventricular Tachycardia from VentricularFibrillation and Treatment Thereof” to Bardy, et al, (all incorporatedherein by reference in their entireties). Particular algorithms fordetection of ventricular fibrillation and malignant ventriculartachycardias can be selected from among the comprehensive algorithms fordistinguishing atrial and ventricular tachyarrhythmias from one anotherand from high rate sinus rhythms that are set forth in the '316, '186,'593 and '593 patents.

The detection algorithms are highly sensitive and specific for thepresence or absence of life threatening ventricular arrhythmias, e.g.,ventricular tachycardia (VT) and ventricular fibrillation (VF). When amalignant tachycardia is detected, high voltage capacitors 156, 158,160, and 162 are charged to a pre-programmed voltage level by ahigh-voltage charging circuit 164. It is generally consideredinefficient to maintain a constant charge on the high voltage outputcapacitors 156, 158, 160, 162. Instead, charging is initiated whencontrol circuit 144 issues a high voltage charge command HVCHG deliveredon line 145 to high voltage charge circuit 164 and charging iscontrolled by means of bi-directional control/data bus 166 and afeedback signal VCAP from the HV output circuit 140. High voltage outputcapacitors 156, 158, 160 and 162 may be of film, aluminum electrolyticor wet tantalum construction.

The negative terminal of high voltage battery 112 is directly coupled tosystem ground. Switch circuit 114 is normally open so that the positiveterminal of high voltage battery 112 is disconnected from the positivepower input of the high voltage charge circuit 164. The high voltagecharge command HVCHG is also conducted via conductor 149 to the controlinput of switch circuit 114, and switch circuit 114 closes in responseto connect positive high voltage battery voltage EXT B+ to the positivepower input of high voltage charge circuit 164. Switch circuit 114 maybe, for example, a field effect transistor (FET) with itssource-to-drain path interrupting the EXT B+ conductor 149 and its gatereceiving the HVCHG signal on conductor 145. High voltage charge circuit164 is thereby rendered ready to begin charging the high voltage outputcapacitors 156, 158, 160, and 162 with charging current from highvoltage battery 112.

High voltage output capacitors 156, 158, 160, and 162 may be charged tovery high voltages, e.g., 700-3150V, to be discharged through the bodyand heart between the electrode pair of subcutaneouscardioversion-defibrillation electrodes 113 and 123. The details of thevoltage charging circuitry are also not deemed to be critical withregard to practicing the present invention; one high voltage chargingcircuit believed to be suitable for the purposes of the presentinvention is disclosed. High voltage capacitors 156, 158, 160 and 162are charged by high voltage charge circuit 164 and a high frequency,high-voltage transformer 168 as described in detail in commonly assignedU.S. Pat. No. 4,548,209 “Energy Converter for Implantable Cardioverter”to Wielders, et al. Proper charging polarities are maintained by diodes170, 172, 174 and 176 interconnecting the output windings ofhigh-voltage transformer 168 and the capacitors 156, 158, 160, and 162.As noted above, the state of capacitor charge is monitored by circuitrywithin the high voltage output circuit 140 that provides a VCAP,feedback signal indicative of the voltage to the timing and controlcircuit 144. Timing and control circuit 144 terminates the high voltagecharge command HVCHG when the VCAP signal matches the programmedcapacitor output voltage, i.e., the cardioversion-defibrillation peakshock voltage.

Control circuit 144 then develops first and second control signalsNPULSE 1 and NPULSE 2, respectively, that are applied to the highvoltage output circuit 140 for triggering the delivery of cardiovertingor defibrillating shocks. In particular, the NPULSE 1 signal triggersdischarge of the first capacitor bank, comprising capacitors 156 and158. The NPULSE 2 signal triggers discharge of the first capacitor bankand a second capacitor bank, comprising capacitors 160 and 162. It ispossible to select between a plurality of output pulse regimes simply bymodifying the number and time order of assertion of the NPULSE 1 andNPULSE 2 signals. The NPULSE 1 signals and NPULSE 2 signals may beprovided sequentially, simultaneously or individually. In this way,control circuitry 144 serves to control operation of the high voltageoutput stage 140, which delivers high energycardioversion-defibrillation shocks between the pair of thecardioversion-defibrillation electrodes 113 and 123 coupled to the HV-1and COMMON output as shown in FIG. 3.

Thus, device 14 monitors the patient's cardiac status and initiates thedelivery of a cardioversion-defibrillation shock through thecardioversion-defibrillation electrodes 113 and 123 in response todetection of a tachyarrhythmia requiring cardioversion-defibrillation.The high HVCHG signal causes the high voltage battery 112 to beconnected through the switch circuit 114 with the high voltage chargecircuit 164 and the charging of output capacitors 156, 158, 160, and 162to commence. Charging continues until the programmed charge voltage isreflected by the VCAP signal, at which point control and timing circuit144 sets the HVCHG signal low terminating charging and opening switchcircuit 114. Typically, the charging cycle takes only fifteen to twentyseconds, and occurs very infrequently. The device 14 can be programmedto attempt to deliver cardioversion shocks to the heart in the mannersdescribed above in timed synchrony with a detected R-wave or can beprogrammed or fabricated to deliver defibrillation shocks to the heartin the manners described above without attempting to synchronize thedelivery to a detected R-wave. Episode data related to the detection ofthe tachyarrhythmia and delivery of the cardioversion-defibrillationshock may be stored in RAM for uplink telemetry transmission to anexternal programmer as is well known in the art to facilitate indiagnosis of the patient's cardiac state. A patient receiving the device14 on a prophylactic basis would be instructed to report each suchepisode to the attending physician for further evaluation of thepatient's condition and assessment for the need for implantation of amore sophisticated implantable cardio-defibrillator device (ICD). Inother embodiments, no storage of episode data will take place.

FIG. 4 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention. Asillustrated in FIG. 4, once control circuit 144 determines the presenceof a malignant cardiac event using the primary detection algorithmdescribed above, Block 400, the present invention generates a secondaryconfirmation of the event detected by the primary detection algorithm,blocks 402-410. In particular, once the primary detection algorithm issatisfied, the present invention utilizes the input generated fromoptical sensor 17 at multiple wavelengths to identify an O₂ variationindex trend, block 402, as described below in detail. According to thepresent invention, the O₂ variation index trend is a measure of thechange in tissue oxygenation and corresponds to the relationship betweenchanges in both the volume of blood at the sensor site and theconcentration of oxygenated hemoglobin (Hb O₂).

Using the results of the generated O₂ variation index trend, adetermination is made as to whether the detected event is associatedwith noise, resulting from patient motion, for example, Block 404. If itis determined that the detected event is associated with noise, deliveryof therapy is withheld, or control of the device is reverted back to theprimary detection algorithm, Block 406. However, if it is determinedthat the detected event is not associated with noise, a determination ismade as to whether the detected event is associated with an unstablerhythm, Block 408, such as ventricular tachycardia or ventricularfibrillation, for example.

If the detected event is not determined to be associated with anunstable rhythm, the result of the primary detection scheme is notconfirmed and delivery of therapy is withheld or control of the deviceis reverted back to the primary detection algorithm, Block 406. If thedetected event is determined to be associated with an unstable rhythm,the primary detection of the malignant event is confirmed, Block 410,and therapy is delivered.

FIG. 5 is a graphical representation of identification of an exemplaryO₂ variation index trend utilized in a method of delivering a therapy ina medical device according to an embodiment of the present invention. Asillustrated in FIG. 5, once control circuit 144 determines the presenceof a cardiac event, such as ventricular tachycardia or ventricularfibrillation, for example, using the primary detection algorithmdescribed above, the presence of the cardiac event is confirmed bydetermining changes in blood oxygenation of the patient. The changes inblood oxygenation are determined using an O₂ variation index trend 200that is identified based on the intensity readings associated with theintensity of the red light emitted by red LED 21 and the infrared lightemitted by infrared LED 23 that is received at photodiode 30.

In particular, in order to identify the O₂ variation index trend 200,both a red light baseline intensity i₀ and an infrared light baselineintensity i₀* is identified from sample outputs received at apredetermined sample rate over a sampling time interval. For example,according to an embodiment of the present invention, sample outputs arereceived at photodiode 30 from red LED 21 and infrared LED 23 at asampling rate of three samples per second over a two second samplingtime interval. Baseline intensity i₀ and baseline intensity i₀* are thendetermined from the sample outputs from red LED 21 and infrared LED 23,respectively. For example, according to an embodiment of the presentinvention, baseline intensity i₀ and baseline intensity i₀* aredetermined, respectively, by setting baseline intensity to equal to theaverage of the sample outputs from red LED 21 over a predetermined timeperiod and setting baseline intensity i₀* equal to the average of thesample outputs from infrared LED 23 over the predetermined time period.

Once the red and infrared baseline intensities i₀ and i₀* have beendetermined, a variation index is determined for each subsequentlyreceived two-wavelength sample output by sensor 17 using an oxygenvariation index equation:

$\begin{matrix}{{{Variation}\mspace{14mu}{index}} = {\frac{i}{i_{0}} - \frac{i^{*}}{i_{0}^{*}}}} & {{Equation}\mspace{20mu} 1}\end{matrix}$where i is the intensity of red light from red LED 21 incident onphotodetector 30 for a given sample and i* is the intensity of infraredlight from infrared LED 21 incident on photodetector 30 for the samesample. In this way, the variation index for each two-wavelength sampleoutput is the difference between the proportion of the red and theinfrared intensity signals with respect to their corresponding baselineintensities. Using the exemplary sampling rate of 3 Hz, three variationindexes are generated each second and are used to determine the O₂variation index trend 200.

It is understood that other relationships between the proportions of thered and infrared intensities to their corresponding baseline intensities

$\frac{i}{i_{0}}\mspace{14mu}{and}\mspace{14mu}{\frac{i^{*}}{i_{0}^{*}}.}$For example, if the proportion of the red intensity signal to thebaseline red intensity

$\frac{i}{i_{0}}$is referred to as the normalized red intensity and the proportion of theinfrared intensity signal to the baseline infrared intensity

$\frac{i^{*}}{i_{0}^{*}}$is referred to as the normalized infrared intensity, Equation 1 mayalternatively be a ratio of the normalized red and infrared intensities,or may be a difference between the unequally weighted red and infrarednormalized intensities.

FIG. 5 includes three oxygen variation index trends identified using theoxygen variation index equation, Equation 1. The first is an exemplaryO₂ variation index trend resulting during normal sinus rhythm 202, thesecond is an exemplary O₂ variation index trend resulting during noise204, such as patient motion for example, and the third is an exemplaryO₂ variation index trend resulting during a malignant cardiac event 206,such as ventricular fibrillation for example. As can be seen in FIG. 5,using the O₂ variation index trend 200 identified according to thepresent invention, the O₂ variation index trend resulting during noise204 tends to exhibit a variability that is significantly greater thanthe variability exhibited by the O₂ variation index trend resultingduring normal sinus rhythm 202, while the O₂ variation index trendresulting during the malignant cardiac event 206 tends to exhibit avariability that is generally equivalent to or less than the variabilityexhibited by the O₂ variation index trend resulting during normal sinusrhythm 202. As described below, the present invention utilizes thesevariation features and others to perform a secondary confirmation of acardiac event determined by a primary detection scheme to be a malignantcardiac event. One such criterion involves crossing of a predeterminedbaseline 210 by a parameter, described in detail below.

FIG. 6 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention. FIG. 7 is anexemplary O₂ variation index trend identified in a method of deliveringa therapy in a medical device according to an embodiment of the presentinvention. As illustrated in FIGS. 6 and 7, once the primary detectionalgorithm is satisfied, Yes in Block 500, the present invention beginswith the initialization of several parameters, such as a corrected O₂index trend, Block 501, which is intended to represent a measure of thechange in tissue oxygenation and will be utilized below in reference toBlock 508.

Once the initialization of parameters is complete, i.e., the correctedO₂ index trend is set equal to zero, the present invention beginscomputing O₂ variation indexes 600 using the optical sample inputs fromoptical sensor 17 at multiple wavelengths and Equation 1, Block 502.Optical sample inputs from sensor 17 are collected and the correspondingO₂ variation indexes 600 are computed using Equation 1 for apredetermined sample collection period. The predetermined samplecollection period may be set as a predetermined period of time, such as5 seconds, for example, or may be set as a predetermined number ofsamples, such as 15, for example. In the an exemplary trend according tothe present invention, the sample rate of 3 Hz is utilized and thesample collection period is set as five seconds, for example, so that 15O₂ variation indexes 600 are determined over each sample collectionperiod.

At the end of the initial sample collection period, an O₂ variationindex trend 602 is identified for the sample collection period, Block503, and a measure of the deviation of each of the acquired O₂ variationindexes 600 occurring during the current sample collection period fromthe O₂ variation index trend 602 is determined, Block 504. According toan embodiment of the present invention, the O₂ variation index trend 602is identified in Block 503 by performing a least square linear fit ofthe acquired O₂ variation indexes 600 during the sample collectionperiod, i.e., the first through the 15^(th) O₂ variation indexesO₂₍₁₎-O₂₍₁₅₎, so that the resulting trend has a start point 604 wherethe first O₂ variation index O₂₍₁₎ is projected onto the O₂ variationindex trend 602 and an endpoint 606 where the last O₂ variation indexO₂₍₁₅₎ is projected onto the O₂ variation index trend 602, i.e., the15^(th) O₂ variation index 600, and the measure of deviation of thecurrent samples from the trend, Block 504, is performed by determiningthe mean square deviation of the O₂ variation indexes 600 in the currentwindow of O₂ variation indexes from the O₂ variation index trend 602.

According to an embodiment of the present invention, the O₂ variationindex trend 602 may be obtained, for example, by an alternativefiltering technique and the measure of the deviation of the indexes 600from the O₂ variation index trend 602 may be determined as the meansquare of the indexes 600 from the filtered index trend.

In order to perform the secondary determination of whether the detectedevent is associated with noise (Block 404 of FIG. 4), once the deviationof the associated O₂ variation indexes 600 from the current O₂ variationindex trend is determined, Block 504, a determination is then made as towhether the deviation is less than a predetermined deviation threshold,Block 505.

If the deviation of the O₂ variation indexes 600 for the current windowof O₂ variation indexes 600 is not less than or is equal to thedeviation threshold, No in Block 505, indicating a likelihood that thedetermined cardiac event may be the result of noise, a determination ismade as to whether a predetermined episode verification time period hasexpired, Block 507. If the episode verification time period has notexpired, the process returns to Block 502 so that the deviation of O₂variation indexes from the O₂ variation index trend, Blocks 503 and 504,is determined for the next window of O₂ variation indexes 600.

In particular, once the deviation of the O₂ variation indexes 600 forthe current window of O₂ variation indexes is determined to be greaterthan the deviation threshold and the episode verification time period,No in Block 507, i.e., 30 seconds for example, has not expired, an O₂variation index trend 608 is determined for the next window of O₂variation indexes, i.e., the window including the 2^(nd) through the16^(th) O₂ variation indexes O₂₍₂₎-O₂₍₁₆₎, so that the resulting trendhas a start point 610 where the second O₂ variation index O₂₍₂₎ isprojected onto the O₂ variation index trend 608 and an endpoint 612where the last O₂ variation index O₂₍₁₆₎ is projected onto the O₂variation index trend 608. The deviation of the O₂ variation indexes 600for that window of O₂ variation indexes O₂₍₂₎-O₂₍₁₆₎ from the O₂variation index trend 608 is determined and compared to the deviationthreshold, Block 505. In this way, the process of the present inventioncontinues to compute the O₂ variation index trend over a moving group ofconsecutive computed O₂ variation indexes including some of the mostrecently acquired samples, with the size of the group of samples beingconsistent with the sample collection period.

The process continues for the next window of O₂ variation indexes 600,and if the deviation of generated O₂ variation indexes 600 from theassociated trends for the subsequent sample collections periodscontinues to be greater than the deviation threshold and therefore theepisode verification time period has expired, the secondary confirmationprocess determines that the cardiac event is most likely related tonoise, and therapy is withheld or control of the device is reverted backto the primary detection algorithm, Block 506.

The episode verification time period may be set at any desired value, sothat the determination of whether the cardiac event is noise related maybe made for a predetermined number of iterations or over a predeterminedtime period, depending on the chosen values for the sample collectionperiod and the episode verification time period. For example, accordingto an embodiment of the present invention, the episode verification timeperiod is set at 30 seconds and the sample collection period is set at 5seconds. In this example, if the optical sensor signals are sampled a 3Hz, ninety O₂ variation indexes are computed over the episodeverification time period, with the associated O₂ variation index trendand deviation being computed every 5 seconds over a moving group of 15consecutive samples. Since the computation of the O₂ variation indextrend begins once 15 variation indexes are computed, the determinationof whether a detected cardiac event is noise related, and if not,whether it is hemodynamically stable, Blocks 509 and 511, is made forseventy-six iterations, over each 30 second episode verification timeperiod.

If the deviation of the O₂ variation indexes for a given window of O₂variation indexes is determined to be less than the deviation threshold,Yes in Block 505, indicating a likelihood that the determined cardiacevent is not the result of noise, the corrected O₂ index trend isincremented by the determined deviation of the current O₂ variationindexes, Block 508, as will be described in detail below. Adetermination is then made as to whether the current generated O₂variation index trend 602 is less than a predetermined baseline value210, Block 509.

According to the present invention, the baseline value 210 associatedwith Block 509 corresponds to a desired deviation associated with therelationship between the proportion of the intensity of red light i fromLED 21 to the baseline intensity i₀ for LED 21, or

$\frac{i}{i_{0}}$of Equation 1, and the proportion of the intensity of infrared light i*from LED 23 to the baseline intensity i₀* for LED 23, or

$\frac{i^{*}}{i_{0}^{*}}$of Equation 1. For example, according to an embodiment of the presentinvention, the baseline value 210 is set as −0.02, corresponding to theproportion of the intensity of infrared light i* from LED 23 to thebaseline intensity i₀* for LED 23 being greater than the proportion ofthe intensity of red light i from LED 21 to the baseline intensity i₀for LED 21.

If the current O₂ variation index trend is not determined to be lessthan the predetermined baseline value 210, No in Block 509, indicatingthat while the determined cardiac event is not likely the result ofnoise, there is a likelihood that the determined cardiac event may benot be associated with an unstable rhythm, the determination is made asto whether the episode verification time period has expired, Block 507.If the episode verification time period has not expired, the processreturns to Block 502 so that the deviation of O₂ variation indexes 600is determined for the next window of O₂ variation indexes, block 503.

If the current O₂ variation index trend is determined to be less thanthe predetermined baseline value 210, indicating that the determinedcardiac event is not likely the result of noise, and there is alikelihood that the determined cardiac event may be associated with anunstable rhythm, a determination is made as to whether the generated O₂variation index trend is sustained, i.e., remains less than thepredetermined baseline value 210 for a predetermined time period, suchas 3 seconds for example, Block 511.

If the generated O₂ variation index trend is not sustained, i.e., notless than the predetermined baseline value 210 for the predeterminedtime period, the determination is made as to whether the episodeverification time period has expired, Block 507. If the generated O₂variation index trend is not sustained and the episode verification timeperiod has not expired, the deviation of the generated O₂ variationindexes over the next window of O₂ variation indexes is determined,Blocks 503 and 504, and the noise determination is repeated.

If the generated O₂ variation index trend is less than the predeterminedbaseline value 210 for the predetermined time period, i.e., thegenerated O₂ variation index trend is sustained, the secondaryconfirmation process confirms the identification of the malignantcardiac event, and therapy is delivered, Block 512.

According to an embodiment of the present invention, the deviationthreshold of Block 505 is determined, for example, by periodicallycomputing O₂ variation indexes using Equation 1 and generating acorresponding oxygen variation index trend during a known period ofmotion-free normal sinus rhythm, such as while the patient is sleeping.The deviation of the oxygen variation indexes generated during normalsinus rhythm from the corresponding oxygen variation index trendgenerated for the sample collection period is then determined using thesame process utilized in Block 503, such as the mean square deviation,for example. Other methods for determining the deviation may be utilizedrather than the mean square deviations, such as the mean of absolutevalues of deviations, for example. The deviation threshold utilized forBlock 505 is then updated by being set equal to the deviation of theoxygen variation indexes from the trends generated during motion-freenormal sinus rhythm, or to a multiple or a fraction of the deviation.

FIG. 8 is an exemplary graphical representation of an O₂ variation indextrend utilized in a method of delivering a therapy in a medical deviceaccording to an embodiment of the present invention. According to anembodiment of the present invention, during the incrementing of thecorrected O₂ variation index trend, Block 508, the endpoints of thecurrent determined O₂ variation index trends are used to calculate acurrent trend value ΔO_(2(n)) associated with the current samplecollection period. For example, as illustrated in FIG. 8, once the O₂variation index trend 800 has been determined, and the first and thelast O₂ variation index samples of the 15 O₂ variation index samplesassociated with the window of O₂ variation indexes 600 are projectedonto the O₂ variation index trend 800 to determine a first index trendtO₂₍₁₎ and a last index trend tO₂₍₁₅₎ associated with the O₂ variationindex trend 800, as described above, the change in index trend ΔO₂₍₁₅₎for the O₂ variation index trend 800 is then determined as thedifference between the first O₂ variation index trend tO₂₍₁₎ and thelast O₂ variation index trend tO₂₍₁₅₎, i.e., tO₂₍₁₅₎−tO₂₍₁₎. Thecorrected O₂ variation index trend cO₂ is then set equal to thedetermined change in the index trend ΔO₂₍₁₅₎.

A next O₂ variation index trend 802 is then determined for thesubsequent window of O₂ variation index samples, i.e., the next O₂variation index sample O₂₍₁₆₎ and the previous n−1 of the O₂ variationindex samples O₂₍₂₎ through O₂₍₁₅₎. The first and the last O₂ variationindex samples O₂₍₂₎ and O₂₍₁₆₎ are then projected onto the O₂ variationindex trend 802 to determine a first index trend tO₂₍₂₎ and a last indextrend tO₂₍₁₆₎ associated with the O₂ variation index trend 802, and thechange in the index trend ΔO₂₍₁₆₎ for the O₂ variation index trend 802is then determined as the difference between the first index trendtO₂₍₂₎ and the last index trend tO₂₍₁₆₎, i.e., tO₂₍₁₆₎−tO₂₍₂₎.

The corrected O₂ variation index trend cO₂ is then incremented in Block508 by being set equal to the sum of the previous corrected O₂ variationindex trend and the product of the inverse of the number of samples inthe window of O₂ variation indexes and the determined change in the O₂variation index trend ΔO₂₍₁₆₎ for the O₂ variation index trend 800 forthe current window of O₂ variation index samples. This process is thenrepeated so that during noise free periods identified in Block 505, thecorrected O₂ variation index trend cO_(2(i)) is incremented for eachwindow of O₂ variation index samples by being set equal to the sum ofthe previous corrected O₂ variation index trend cO_(2(i−1)) and theproduct of the inverse of the number of samples n in the samplecollection period and the determined change in the O₂ variation indextrend ΔO_(2(i)) for the O₂ variation index trend associated with thecurrent window of O₂ variation index samples, indicated by the followingequation:cO _(2(i)) =cO _(2(i−1))+1/n(ΔO _(2(i)))  Equation 2

According to another embodiment of the present invention, once the firstsample collection period, such as 0-5 seconds for example, has expiredand the corresponding O₂ variation index trend 800 has been determined,15 O₂ variation index trend values tO₂₍₁₎ through tO₂₍₁₅₎ along the O₂variation index trend 800 are identified by projecting the location ofeach of the 15 O₂ variation index samples for the sample collectionperiod onto the O₂ variation index trend 800. The change in the O₂variation index trend ΔO₂₍₁₅₎ for the O₂ variation index trend 800 isthen determined as the difference between the first index trend tO₂₍₁₎and the last index trend tO₂₍₁₅₎, i.e., tO₂₍₁₅₎−tO₂₍₁₎.

An O₂ variation index trend 802 is then determined for the next windowof O₂ variation index samples, and corresponding trend values tO²⁽²⁻¹⁶⁾along the O₂ variation index trend 802 are determined by projecting thelocation of each of the O₂ variation index samples onto the O₂ variationindex trend 802 as described above. The change in the O₂ variation indextrend ΔO₂₍₁₆₎ for the O₂ variation index trend 802 is then determined asthe difference between the first index trend tO₂₍₂₎ and the last indextrend tO₂₍₁₆₎, i.e., tO₂₍₁₆₎−tO₂₍₂₎. The corrected O₂ variation indextrend is then incremented by being set equal to the sum of the change inthe current O₂ variation index trend ΔO₂₍₁₆₎ and the product of theinverse of the number of samples n in window of O₂ variation indexes andthe change in the O₂ variation index trend ΔO₂₍₁₅₎ determined for theprevious sample collection period, indicated by the equationcO₂₍₁₆₎=1/n(ΔO₂₍₁₅₎)+ΔO₂₍₁₆₎.

An O₂ variation index trend 804 is then determined for the next windowof O₂ variation index samples, and corresponding trend values tO²⁽³⁻¹⁷⁾along the O₂ variation index trend 804 are determined by projecting thelocation of each of the O₂ variation index samples associated with thewindow onto the O₂ variation index trend 804. The change in the O₂variation index trend ΔO₂₍₁₇₎ for the O₂ variation index trend 804 isthen determined as the difference between the first O₂ variation indextrend tO₂₍₃₎ and the last O₂ variation index trend tO₂₍₁₇₎, i.e.,tO₂₍₁₇₎−tO₂₍₃₎. The corrected O₂ variation index trend cO₂₍₁₇₎ is thenincremented by being set equal to the sum of the product of the inverseof the number of samples n in the sample collection period and thechange in the O₂ variation index trend ΔO₂₍₁₅₎ determined for the firstO₂ variation index trend 800, the product of the inverse of the numberof samples n in the sample collection period and the change in the O₂variation index trend ΔO₂₍₁₆₎ determined for the previous O₂ variationindex trend 802 and the change in the O₂ variation index trend ΔO₂₍₁₇₎for the current O₂ variation index trend 804, indicated by the equation:cO₂₍₁₇₎₌1/n(ΔO₂₍₁₅₎)+1/n(ΔO₂₍₁₆₎)+ΔO₂₍₁₇₎.

Once the corrected O₂ variation index trend for three noise free samplecollections have been determined, an initialization period for thecorrected O₂ variation index trend incrementation in Block 508 iscompleted, and the corrected O₂ variation index trend cO₂ is updated forsubsequent windows of O₂ variation index samples identified by thedetermined value of the last O₂ variation index trend corresponding tothe endpoint of the most recent determined O₂ variation index trendusing the following equation:cO _(2(i+n)) =cO _(2(i+n−2))+1/n(ΔO _(2(i+n−1)))+ΔO _(2(i+n))  Equation3where i is the last O₂ variation index trend corresponding to theendpoint of the most recent determined O₂ variation index trend,cO_(2(i+n−2)) is the corrected O₂ variation index trend associated withthe window of O₂ variation index samples occurring two sample collectionperiods prior to the current window of O₂ variation index samples, 1/nis the inverse of the number of samples n in the sample collectionperiod, (ΔO_(2(i+n−1))) is the corrected O₂ variation index trendassociated with the window of O₂ variation index samples occurring onesample collection period prior to the current window of O₂ variationindex samples, and ΔO_(2(i+n)) is the change in the current O₂ variationindex trend.

In both embodiments of the present invention associated with Equations 2and 3, since the corrected O₂ variation index trend is incremented onlyfor those sample collection periods that are determined to be noisefree, Yes in Block 505, the present invention accounts for those periodswhen the O₂ variation indexes are likely associated with noise andadjusts the total O₂ variation index trend accordingly by notincrementing the corrected O₂ variation index trend when noise islikely, i.e., when the deviation is not less than the deviationthreshold, No in Block 505.

FIG. 9 is a flow chart of a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention. FIG. 10 isan exemplary graphical representation of an O₂ variation index trendutilized in a method of delivering a therapy in a medical deviceaccording to an embodiment of the present invention. As illustrated inFIGS. 9 and 10, according to an embodiment of the present invention,once the first sample collection period, i.e., 0-5 seconds, has expired,and therefore both the associated O₂ variation index trend 900, Block503, and the deviation of the associated samples from the trend, Block504, over that period have been determined and utilized to determinethat the sample is likely to be noise free, Yes in Block 505, the lasttrend value tO₂₍₁₅₎ along the corresponding O₂ variation index trend 900is determined by projecting the location of the last O₂ variation indexsample of the window O²⁽¹⁻¹⁵⁾ onto the O₂ variation index trend 900.

For the initial sample collection period, the corrected O₂ variationindex trend cO₂ is incremented in Block 508 by being set equal to thelast trended value tO₂₍₁₅₎, which is then utilized as the generatedcorrected O₂ variation index trend cO₂ for the determination of Block509.

The window of O₂ variation index samples shifts to include the next O₂variation index sample O₂₍₁₆₎ and the previous n−1 O₂ variation indexsamples O²⁽²⁻¹⁵⁾ from the previous window of samples. The current O₂variation index samples O²⁽²⁻¹⁶⁾ are then used to determine the next O₂variation index trend 902, with the value of the last O₂ variation indexsample O₂₍₁₆₎ being projected onto the current O₂ variation index trend902 to generate a corresponding last trended value tO₂₍₁₆₎, which isthen utilized as the corrected O₂ variation index trend in thedetermination of Block 509, and so forth. An O₂ variation index trend isthen determined for the next window of O₂ variation index samples andthe corresponding last trended value is determined by projecting thelocation of the last or most recent O₂ variation index sample onto theO₂ variation index trend, and so forth.

The process continues as described on a sample by sample basis until theeffects of noise cause the O₂ variation index samples to deviate fromthe O₂ variation index trend so that the deviation becomes greater thanthe deviation threshold, No in Block 505. Once the deviation of the O₂variation indexes is determined not to be less than the deviationthreshold, indicating a likelihood that the determined cardiac event isthe result of noise, a corrected O₂ variation index offset, whichoperates to keep a running account of the non-noise free periods, isupdated, Block 510. In particular, as the window of O₂ variation indexsamples continues to be shifted to include a next O₂ variation indexsample and the previous n−1 samples, and the corrected O₂ variationindex trend continues to be updated accordingly, the leading edge of thewindow of O₂ variation index samples may begin to advance within a noiseportion 906. Once the window of O₂ variation index samples advances farenough within the noise portion 906, the deviation of the samples in thecurrent sample window will become greater than the deviation threshold,No in Block 505, and therefore the value of the corrected O₂ variationindex trend is held equal to the last corrected O₂ variation index trendthat was not associated with noise.

Assuming that the window of O₂ variation index samples that initiallydeviates from the corresponding O₂ variation index trend to beindicative of noise occurs at O₂ variation index sample O_(2(x)), whichcorresponds to trended O₂ variation index sample tO_(2(x)), and thecorrected O₂ variation index is therefore no longer updated, the offsetis updated in Block 510 by being set equal to the difference between thecurrent trended O₂ variation index tO_(2(x)) and the corrected O₂variation index associated with the last window of samples determined tobe noise free, illustrated in FIG. 10 by tO_(2(x))−cO_(2(x−1)). Theupdating of the offset continues, with the offset being updated in Block510 for each subsequent window of samples by continuing to determine thedifference between the current trended O₂ variation index tO_(2(x+m))and the corrected O₂ variation index associated with the last window ofsamples determined to be noise free, until another window of noise freeO₂ variation index samples is received, Yes in Block 505. The updatingof the offset in Block 510 as the window of samples advances duringdetection of the noise 906 can therefore be summarized generally by thefollowing equation:offset=tO _(2(i+m)) −cO _(2(i−1))  Equation 4where i is the first instance that noise is detected for a given noiseperiod, m is the subsequent consecutive samples during this period ofnoise, and i−1 corresponds to the last corrected O₂ variation indextrend value for which the associated window of samples was determined tobe noise free immediately prior to the trend value for which theassociated window of samples was determined to be likely corrupted bynoise.

Assuming the next noise free O₂ variation index sample is received forthe O₂ variation index sample associated with O₂ variation index trendtO_(2(y)), for example, the incrementing of the corrected O₂ variationindex trend in Block 508 then continues and the updating of the offsetin Block 510 is suspended. As a result, the offset was last updated forthe previous sample O_(2(y−1)) by being set equal to the differencebetween the value of the O₂ variation index trend tO_(2(y−1)) generatedduring the window of samples occurring just prior to the initial noisefree window of samples and the value of the corrected O₂ variation indexcO_(2(x−1)) associated with the last window of samples determined to benoise free.

The corrected O₂ variation index trend is then incremented in Block 508by the difference between the current noise free trend value tO_(2(y))and the sum of previously determined offsets, which in the example wouldbe the offset associated with noise period 906, illustrated bytO_(2(y−1))−cO_(2(x−1)). It may also be noted that since noise period906 is the first noisy period in the example, the offset equals thetotal change in the O₂ variation index trend during the period of noisetO_(2(y−1))−cO_(2(x−1)). The incrementing of the corrected O₂ variationindex trend continues with the window of O₂ variation index samplesbeing shifted to include the next O₂ variation index sample O_(2(y+m))and the previous 14 O₂ variation index samples starting from O₂variation index sample O_(2(y+m−14)), so that the subsequent windows ofsamples are used to determine the next O₂ variation index trends, andthe last O₂ variation index sample O_(2(y+m)) is projected onto thecurrent O₂ variation index trend to generate a corresponding trend valuetO_(2(y+m)). The corrected O₂ variation index trend is then incrementedin Block 508 by subtracting the offset updated during the previous noiseperiod 906 from the current trend value tO_(2(y+m)).

Assuming, for example, that the current window of O₂ variation indexsamples subsequently remains noise free for a noise free period of time908 and then deviates from the corresponding O₂ variation index trend,No in Block 505, to be indicative of a next noise portion 910 at O₂variation index sample O_(2(z)), which corresponds to trended O₂variation index sample tO_(2(z)), the corrected O₂ variation index is nolonger incremented for trended O₂ variation index sample tO_(2(z)),while the offset is updated in Block 510 by being set equal to thedifference between the current value of the non-noise free trended O₂variation index sample tO_(2(z)) and the value of the corrected O₂variation index associated with the last window of samples determined tobe noise free, i.e cO_(2(z−1)). The updating of the offset continues foreach subsequent window of samples by taking the difference between thevalue of the trended O₂ variation index generated for the currentnon-noise free window of samples and the value of the corrected O₂variation index associated with the last window of samples determined tobe noise free, until the next noise free O₂ variation index sample isreceived.

For example, assuming the next noise free O₂ variation index sample isreceived for the O₂ variation index sample associated with trended O₂variation index sample tO_(2(w)), the incrementing of the corrected O₂variation index trend in Block 508 then continues and updating of theoffset in Block 510 is suspended. As a result, the offset was lastupdated for the previous sample tO_(2(w−1)) by being set equal to thedifference between the value of the O₂ variation index trend tO_(2(w−1))generated during the window of samples occurring just prior to theinitial noise free window of samples and the value of the corrected O₂variation index associated with the last window of samples determined tobe noise free, cO_(2(z−1)). It should be noted that the offset at thispoint equals the sum of the changes observed during the two precedingnoisy periods 906 and 910,[tO_(2(w−1))−tO_(2(z−1))]+[tO_(2(y−1))−tO_(2(x−1))].

The corrected O₂ variation index trend is then computed as thedifference between the current noise free O₂ variation index trend valuetO_(2(w)) and the current offset. In this way, the corrected O₂variation index trend is updated for subsequent windows of O₂ variationindex samples by subtracting the sum of the previous changes in the O₂variation index trend values associated with noise from the current O₂variation index trend value, described generally by the followingequation:cO _(2(i−n+1)) =tO _(2(i−n+1))−Σoffset  Equation 5where i is the current O₂ variation index trend value and n is thenumber of samples in a sampling window. Σoffset represents the currentoffset at any point in time, where the summation, Σ, is indicative ofthe fact that the current offset represents the sum of individualoffsets accumulated during each individual period of noise since thestart of the flow chart As can be seen in Equation 5, the incrementingof the corrected O₂ variation index trend does not begin until the firstn samples associated with the first window of samples 900 are received.

As illustrated in FIG. 10, each period of noise 906, 910 associated withthe O₂ variation index trend includes a start point 920 and an endpoint922. Ideally, noise will be detected, No in Block 505, when the endpointof the associated O₂ variation index trend is located at or just beyondthe start point 920 of the period of noise 906, 910, and willsubsequently no longer be detected, Yes in Block 505, when thesubsequent endpoint of the associated O₂ variation index trend islocated at or just beyond the endpoint 922. However, as can be in theembodiment described above in reference to FIG. 10, when the endpointsof the O₂ variation index trends are used in the determination of boththe incrementing of the corrected O₂ variation index trend, Block 508,and the updating of the offset, Block 510, the window of O₂ variationindex samples will be located beyond the start point 920 and within thenoise portion when the noise is initially identified, No in Block 505,and beyond the endpoint 922 and outside the noise period when noise issubsequently no longer detected, Yes in Block 505.

According to an embodiment of the present invention, therefore, in orderto increase the likelihood that the offset will correspond to the actualperiod of noise, the present invention utilizes a predetermined trendvalue other than the leading endpoint of the O₂ variation index trend.Rather than projecting on the first and the last sample of each of the nsamples in the samples of windows to obtain the first O₂ variation indextrend value and the last O₂ variation index trend value for eachgenerated O₂ variation index trend as described above, each samplewithin the window of samples is projected onto the O₂ variation indextrend to generate n O₂ variation index trend values so that any one ofthe trend values can then be utilized during the incrementing andupdating of the corrected O₂ variation index trend and the offset,respectively.

For example, once the initial O₂ variation index trend 900 has beencomputed, Block 503 and the deviation of the samples from the O₂variation index trend 900 has been determined, Block 504, resulting in adetermination that the sample is not likely associated with noise, Yesin Block 505, the corrected O₂ variation index trend cO₂ is incrementedin Block 508 by being set equal to a predetermined one of the 15 trendvalues located between the first trend value tO₂₍₁₎ associated with thefirst O₂ variation index sample of the window and the last trend valuetO₂₍₁₅₎ associated with the last O₂ variation index sample of the windowO₂₍₁₅₎. According to an embodiment of the present invention, thecorrected O₂ variation index trend cO₂ is incremented by being set equalto the central trend value, i.e., O₂ variation index trend value tO₂₍₈₎.

The process continues with the window of O₂ variation index samplesbeing shifted to include the next O₂ variation index sample O₂₍₁₆₎ andthe previous 14 O₂ variation index samples from the previous window ofsamples O₂₍₂₎ through O₂₍₁₅₎. The current window of samples O²⁽²⁻¹⁶⁾ isthen used to determine the next O₂ variation index trend 902, and thecentral O₂ variation index sample O₂₍₉₎ is projected onto the current O₂variation index trend 902 to generate a corresponding central trendvalue tO₂₍₉₎. The corrected O₂ variation index trend is then incrementedin Block 508 by being set equal to the central trend value tO₂₍₈₎. Theprocess continues as described above using the predetermined trend valuein place of the last O₂ variation index trend value to perform theincrementing and updating of the corrected O₂ variation index trend andthe offset, respectively.

FIG. 11 is an exemplary graphical representation of generation of acorrected O₂ variation index trend offset utilized in a method ofdelivering a therapy in a medical device according to an embodiment ofthe present invention. According to another embodiment of the presentinvention, rather than using a single trended value during theincrementing and updating of the corrected O₂ variation index trend andthe offset, respectively, multiple values may be utilized. For example,as illustrated in FIG. 11, the last or leading end O₂ variation indextrend is utilized during the incrementing of the O₂ variation indextrend and a predetermined O₂ variation index trend value is utilizedduring the updating of the offset. The predetermined O₂ variation indextrend value is chosen to increase the likelihood that the offset willcorrespond to the actual period of noise, such as the midpoint of the O₂variation index trend, for example. In this way, the incrementing of thecorrected O₂ variation index trend is performed using the leadingendpoint of the determined O₂ variation index trend during the initialnoise free period associated with O₂ variation index trends 900, 902,and so forth, and the process continues as described on a sample bysample basis until the effects of noise cause the O₂ variation indexsamples to deviate from the O₂ variation index trend, No in Block 505.

Assuming again that the window of O₂ variation index samples thatinitially deviates from the corresponding O₂ variation index trend to beindicative of noise occurs at O₂ variation index sample O_(2(x)), whichcorresponds to O₂ variation index trend tO_(2(x)), incrementing of thecorrected O₂ variation index is therefore suspended. It should be notedthat while computing an offset in Block 510, the corrected O₂ variationindex trend values belong to the immediately prior sampling window.During the updating of the offset in Block 510, the O₂ variation indextrend offset is updated by determining the difference between theleading O₂ variation index trend value tO_(2(x−8)) of the current noisecorrupted O₂ variation index trend tO_(2(x)) and the corrected O₂variation index trend cO_(2(x−9)) determined immediately prior to thecentral O₂ variation index trend tO_(2(x−8)).

The process continues for subsequent windows of samples, with thedetermination of whether the current window of samples are corrupted bynoise being made in Block 505 and the updating of the offset in Block510 being made based on the predetermined trend value, until the nextnoise free O₂ variation index sample is identified, Yes in Block 505, sothat the offset is updated for each window of samples as set forthgenerally by the following equation:offset=tO _(2(i+m−d)) −cO _(2(i−1))  Equation 6where i is the first instance that noise is detected for a given noiseperiod, m is the next sample, d corresponds to the predetermined trendvalue associated with the current noise corrupted window of O₂ variationindex samples, and i−1 corresponds to the immediate last trend valueassociated with the window of samples determined to be noise free priorto the predetermined trend value associated with the current noisecorrupted window of samples.

For example, if the next noise free O₂ variation index sample isreceived at O₂ variation index trend tO_(2(y)), the offset was thereforelast updated during the previous trended O₂ variation index sampletO_(2(y−1)) by being set equal to the difference between the central O₂variation index trend value tO_(2(y−9)) of the current noise corruptedO₂ variation index trend and the corrected O₂ variation index trendcO_(2(x−9)) determined prior to the central O₂ variation index trendtO_(2(x−8)). Incrementing of the corrected O₂ variation index trendcO_(2(y)) is resumed in Block 508 by subtracting the offset 907 from thecurrent O₂ variation index trend tO_(2(y)).

The process continues during over m windows of O₂ variation indexsamples occurring over the subsequent noise free portion 908, with thecorrected O₂ variation index being incremented, Block 508, bysubtracting the offset 907 from the current trend value tO_(2(y+m)).Assuming, for example, that the current window of O₂ variation indexsamples subsequently remains noise free for a period of time associatedwith the noise free period 908 and then deviates from the correspondingO₂ variation index trend, No in Block 505, to be indicative of a nextnoise portion 910 at O₂ variation index trend tO_(2(z)), incrementing ofthe corrected O₂ variation index is therefore suspended and updating ofthe offset in Block 510 resumes.

During the updating of the offset, the O₂ variation index trend offsetis updated by determining the difference between the central O₂variation index trend value tO_(2(z−8)) of the current noise corruptedO₂ variation index trend tO_(2(z)) and the corrected O₂ variation indextrend cO_(2(z−9)) determined immediately prior to the central O₂variation index trend tO_(2(z−8)). The process continues for subsequentwindows of samples, with the determination of whether the current windowof samples are corrupted by noise being made in Block 505 and theupdating of the offset in Block 510 being made based on thepredetermined trend value using Equation 6, as in the previous noiseperiod 906 until the next noise free O₂ variation index sample isidentified, Yes in Block 505.

For example, if the next noise free O₂ variation index sample isreceived at O₂ variation index trend tO_(2(w)), the offset was thereforelast updated for the previous O₂ variation index trend tO_(2(w−1)) bybeing set equal to the difference between the central O₂ variation indextrend tO_(2(w−9)) associated with the previous O₂ variation index trendand the last incremented corrected O₂ variation index trend cO_(2(z−9))prior to the central O₂ variation index trend cO_(2(w−9)). Incrementingof the corrected O₂ variation index trend cO_(2(w)) is resumed in Block508 by subtracting the sum of the offsets 907 and 909 from the currentO₂ variation index trend tO_(2(w)).

The process continues during over m windows of O₂ variation indexsamples occurring over the subsequent noise corrupted and noise freeportions, with the offset being updated by determining the currentoffset using Equation 6 and the incrementing of the corrected O₂variation index in Block 508 being performed by subtracting the sum ofthe prior determined offsets from the current O₂ variation index trend,set forth generally in Equation 5 above.

FIG. 12 is an exemplary graphical representation of an O₂ variationindex trend utilized in a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention. FIG. 13 is aflow chart of a method of delivering a therapy in a medical deviceaccording to an embodiment of the present invention. As illustrated inFIGS. 12 and 13, according to an embodiment of the present invention, azone 920 for identifying when the event is most likely associated withan unstable hemodynamic event, such as ventricular tachycardia orventricular fibrillation, is defined based on a non-physiologic eventthreshold 921 and a normal sinus rhythm threshold limit 922, so thatonce the O₂ variation index trend is determined to be less than thebaseline value 210, a determination is made as to whether the O₂variation index trend is within the VT/VF zone 920, Block 513.

In particular, for example, a determination is made as to whether theslope of the O₂ variation index trend, determined based on two of theknown trended values of the O₂ variation index trend, such as the firstand the last trended value, for example, is either greater than theslope of threshold 921 or less than the slope of threshold 922, andtherefore outside the VT/VF zone 920, Block 513. If the O₂ variationindex trend is determined to be outside the VT/VF zone, No in Block 513,the current stored slope values are cleared and the determination as towhether the episode verification time period has expired is made, Block507, described above. If the O₂ variation index trend is determined tobe within the VT/VF zone, Yes in Block 513, a determination is made asto whether the O₂ variation index trend is sustained, i.e., remainswithin the zone 920 for a predetermined time period, such as over sixsamples or two seconds, for example, Block 515.

If the O₂ variation index trend is not sustained, the process returns toBlock 502 so that the deviation of the O₂ variation index samples fromthe O₂ variation index trend, Blocks 503 and 504 is determined for thenext window of O₂ variation index samples, described above. If the O₂variation index trend is sustained within the VT/VF zone 920 for thepredetermined period of time, the secondary confirmation processconfirms the identification of the malignant cardiac event, and therapyis delivered, Block 512, and the current stored slope values arecleared.

Both threshold 921, which corresponds to abrupt changes in the slope ofthe O₂ variation index trend indicative of non-physiological events,such as a change in posture for example, and threshold 922, whichcorresponds to normal sinus rhythm, are programmable. According to anexemplary embodiment of the present invention, threshold 921 correspondsto the O₂ variation index trend crossing the baseline value 210 in fiveseconds or less, so that threshold 921 corresponds to a slope of 0.004(i.e., 0.02 divided by 5 seconds), and threshold 922 corresponds to theO₂ variation index trend crossing the baseline value 210 in 20 or moreseconds, so that threshold 922 corresponds to a slope of 0.001 (i.e.,0.02 divided by 20 seconds).

FIG. 14 is an exemplary graphical representation of an O₂ variationindex trend utilized in a method of delivering a therapy in a medicaldevice according to an embodiment of the present invention. FIG. 15 is aflow chart of a method of delivering a therapy in a medical deviceaccording to an embodiment of the present invention. As illustrated inFIGS. 14 and 15, a fast VT threshold 923 is included within the VT/VFzone 920 in order to discriminate VT from VF events, with the eventbeing identified as a fast VT event when the O₂ variation index trend islocated between threshold 922 and threshold 923, and as a VF event whenthe O₂ variation index trend is located between threshold 921 andthreshold 923.

According to another embodiment of the present invention, a slow VTthreshold 925 may also be included in order to discriminate betweennormal sinus rhythm and slow VT events, with O₂ variation index trendsthat are sustained between threshold 925 and threshold 922 beingidentified as associated with a slow VT event.

In particular, once the O₂ variation index trend is determined to beless than the baseline value 210, Yes in Block 509, and the O₂ variationindex trend is determined to be within the VT/VF zone 920, Yes in Block513, as described above, a determination is made as to whether the O₂variation index trend is determined to be sustained within the VT/VFzone 920 for a predetermined time period, Block 515, such as 3-5samples, for example. If the O₂ variation index trend is not sustainedVT/VF, No in Block 515, the process returns to Block 502 so that thedeviation of the O₂ variation index samples from the O₂ variation indextrend, Blocks 503 and 504 is determined for the next window of O₂variation index samples, described above.

If the O₂ variation index trend is sustained VT/VF, Yes in Block 515,the event is identified as a VF event if the O₂ variation index trend islocated between threshold 921 and threshold 923, and as a VT event ifthe O₂ variation index trend is located between threshold 923 andthreshold 922, Block 517. Once the classification of the event isdetermined in Block 517, delivery of the therapy is adjustedaccordingly, Block 519, and the current stored slope values are cleared.

According to an embodiment of the present invention, slow VT threshold925 may also be included in order to discriminate between normal sinusrhythm and slow VT events. In particular, if the O₂ variation indextrend is not determined to be within the VT/VF zone, No in Block 513, adetermination is made as to whether the O₂ variation index trend islocated between threshold 925 and threshold 922. If the O₂ variationindex trend is located between threshold 925 and threshold 922, theevent is classified as a slow VT event and the classification may bestored for future reference.

Similar to threshold 921 and threshold 922, both threshold 923 andthreshold 925 are programmable. According to an exemplary embodiment ofthe present invention, threshold 923 corresponds to the O₂ variationindex trend crossing the baseline value 210 in twelve seconds or less,so that threshold 923 corresponds to a slope of 0.0017 (i.e., 0.02divided by 12 seconds), and threshold 925 corresponds to the O₂variation index trend crossing the baseline value 210 in 28 or moreseconds, so that threshold 925 corresponds to a slope of 0.0007 (i.e.,0.02 divided by 28 seconds).

As described above, when noise is detected, a correction can be made byreferring to the corrected O₂ variation index trend at a prior instantwhen the O₂ variation index trend was unaffected by noise. Since thedetermination of noise by measuring deviation of the O₂ variation indexsamples from the O₂ variation index trend is done over a window ofmultiple samples it may be necessary to look back substantially morethan a single, immediately prior sample, which may lead to a delay inthe determination of the presence of noise from its actual moment ofonset. According to an embodiment of the present invention, in order tomitigate any error due to such a delay, the corrected O₂ variation indextrend consists of multiple values each corresponding to one samplewithin the sampling window for the determination of the O₂ variationindex trend, such that when noise is detected in a sampling window, anearly value from a previously determined group of values of thecorrected O₂ variation index trend can be referred to as one belongingto a noise-free period. This particular value of the corrected O₂variation index trend can then be used for computing an offset tocorrect further O₂ variation index trend.

In this embodiment, for each sampling window, a group of corrected O₂variation index trend values are computed and stored in device 14, eachvalue corresponding to one sample of the O₂ variation index within thewindow. The method of computation of the value of the corrected O₂variation index trend depends on the computed deviation during thesampling window, Block 505 of FIG. 9, and, in case of a large deviationindicative of noise, No in Block 505 of FIG. 9, all the corrected O₂variation index trend values for the sampling window are set to thevalue of the corrected O₂ variation index trend corresponding to apredetermined sample of a predetermined prior sampling window such asthe first sample, labeled i−1, of the sampling window. In case of alarge deviation, No in Block 505 of FIG. 9, the offset is also updatedin Block 510 by assigning to it the difference between the value of theO₂ variation index trend corresponding to a predetermined sample, suchas the sample labeled i+n of the sampling window, and the value of thecorrected O₂ variation index trend corresponding to a predeterminedsample of a predetermined prior sampling window, such as the firstsample, labeled i−1, of the sampling window. Such computation may becontinued for every sample of the O₂ variation index trend where thedeviation is large, No in Block 505 of FIG. 9, according to theequation:offset=tO _(2(m)) −cO _(2(k)) ^(−p)  Equation 7where tO_(2(m)) is the O₂ variation index trend value corresponding tothe predetermined sample m within the current sampling window andcO_(2(k)) ^(−p) is the value of the corrected O₂ variation index trendcorresponding to the predetermined sample k within a prior samplingwindow that precedes the current sampling window by a predeterminednumber of windows indicated by p. The value of the offset so computed isused during the next noise-free sampling window.

In case of a small deviation, Yes in 505 of FIG. 9, indicative of anoise-free sampling window, the corrected O₂ variation index trend iscomputed corresponding to each sample within the interval by taking thedifference between the O₂ variation index trend corresponding to eachsample and the offset computed during the last period of noise.

Since this method depends on the corrected O₂ variation index trend froma prior sampling window, it requires initialization of the corrected O₂variation index trend for a certain number of initial sampling windows,the number being same as the predetermined value, p. For each of thesewindows if the deviation of the O₂ variation index samples from its O₂variation index trend is above its threshold for the determination ofnoise, each value of the corrected O₂ variation index trend in thatsampling window is set to zero otherwise each value of the corrected O₂variation index trend is set to its corresponding value of the O₂variation index trend.

Determination of the hemodynamic status may be improved by furtheranalyzing the slope of the O₂ variation index trend. The slope beingreferred to here is the slope of the O₂ variation index trend line withthe time axis which may also be referred to as the rate of change of theO₂ variation index trend. Such analysis of the slope will enabledetermination of the degree of the perfusion loss which may vary betweentolerated ventricular tachyarrhythmia, non-tolerated ventriculartachyarrhythmia and ventricular fibrillation. A slope is computed foreach sampling window, of 5 second duration consisting of 15 samples forexample, and a certain number of the most recent values of it are storedin the device memory. The slope may be defined as the difference betweenthe first and the last O₂ variation index trend values, for exampletO₂₍₁₅₎−tO₂₍₁₎ for the sampling window consisting of the 1^(st) throughthe 15^(th) sample. It may also be defined as the ratio of thedifference between the first and the last O₂ variation index trendvalues and the time interval between them. For yet another definition ofthe slope, the difference and the ratio defined above may be computedover a plurality of subsections within a sampling window and can befurther combined to derive a composite slope parameter. A plurality ofthe slope values so computed are stored in the device memory.

According to an embodiment of the present invention, the range of valuesof the slope of the corrected O₂ variation index trend is divided intopredetermined groups corresponding to various cardiac rhythms and apredetermined group of values considered non-physiologic. For example asigned slope value smaller than −0.007 per second, called thephysiologic limit, may be considered non-physiologic and any signedslope value larger than −0.00007 per second, called the sinus limit, maybe considered to correspond to a hemodynamically stable, benign cardiacrhythm. Any intermediate slope value between these two limits may beconsidered to correspond to VT or VF. Alternatively, the range of thoseintermediate slope values may be further subdivided with a hemodynamicstability limit demarcating the boundary separating a hemodynamicallystable VT, also called the tolerated VT, from a hemodynamically unstableVT, also called the non-tolerated VT, and VF. The physiologic, sinus andthe hemodynamic stability limits also may be patient-specific anddetermined based on tests such as defibrillation threshold test andother electrophysiologic tests.

The slope of the corrected O₂ variation index trend is compared againstthe predetermined physiologic, sinus and the hemodynamic stabilitylimits to determine the underlying cardiac rhythm and the hemodynamicstatus of the patient, such as a sinus rhythm, a stable VT, an unstableVT or VF or a non-physiologic signal. If it is determined that thecorrected O₂ variation index trend does not correspond to VT or VF, Noin Block 513 of FIG. 13, any value of the slope stored in the device 14is removed, and a determination is made as to whether a predeterminedepisode verification time period has expired, Block 507. If the episodeverification time period has not expired, the process returns to Block502 so that the deviation of O₂ variation indexes from the O₂ variationindex trend, Blocks 503 and 504, is determined for the next window of O₂variation indexes 600.

If on the other hand it is determined that the corrected O₂ variationindex trend corresponds to VT or VF, Yes in Block 513 of FIG. 13, adetermination is also made as to whether it is sustained by collectingadditional samples of O₂ variation index, Block 502 in FIG. 13, andrepeating the subsequent steps for a predetermined number of samples,for example 6, or a predetermined duration, for example 2 seconds, toarrive at the same conclusion, Yes in Block 513. The slope value fromeach pass through the flow chart during such sustainabilitydetermination, 515 in FIG. 13, is stored in the device 14.

The consistency of the corrected O₂ variation index trend slope isdetermined by computing the difference between the maximum and theminimum values of the slopes or the standard deviation or the varianceof the values of the slopes. If such values are smaller than apredetermined limit, the rhythm is classified in Block 517 based on theaverage or the minimum signed value of the slope as either VT and VF orfurther classified as stable VT or unstable VT and VF and a therapy isdetermined based on it, Block 519. The stored slope values are removedfrom the device subsequently, Block 523.

While a particular embodiment of the present invention has been shownand described, modifications may be made. It is therefore intended inthe appended claims to cover all such changes and modifications, whichfall within the true spirit and scope of the invention.

1. A medical device comprising: a plurality of sensors configured tosense cardiac signals; a physiologic sensor sensing physiologic signalsconfigured to generate a plurality of oxygen (O₂) variation indexsamples corresponding to the sensed signals; and a microprocessorconfigured to: detect a cardiac event in response to the sensed cardiacsignals; compute an O₂ variation index trend associated with apredetermined number of O₂ variation index samples of the plurality ofO₂ variation index samples in response to the detected cardiac event;determine, in response to the O₂ variation index trend being less than avariation index trend threshold, whether a rate of change of the O₂variation index trend is within a predetermined cardiac event zonedefined by a first zone threshold having a first rate of changecorresponding to the O₂ variation index trend becoming less than thevariation index trend threshold in a first period of time and a secondzone threshold having a second rate of change corresponding to the O₂variation index trend becoming less than the variation index trendthreshold in a second period of time greater than the first period oftime; and confirm the determined detected cardiac event in response toboth the computed O₂ variation index trend being less than the variationindex trend threshold and the rate of change of the O₂ variation indextrend being within the predetermined cardiac event zone.
 2. The deviceof claim 1, wherein the O₂ variation index trend corresponds to ameasure of change in tissue oxygenation and includes a start pointcorresponding to a first O₂ variation index sample of the predeterminednumber of O₂ variation index samples and an endpoint corresponding to asecond O₂ variation index sample of the predetermined number of O₂variation index samples.
 3. The device of claim 1, wherein themicroprocessor is configured to determine a deviation of thepredetermined number of O₂ variation index samples from the O₂ variationindex trend.
 4. The device of claim 1, wherein the microprocessor isconfigured to determine whether the rate of change is one of greaterthan the first zone threshold and less than the second zone threshold.5. The device of claim 1, wherein the microprocessor is configured todetermine whether the rate of change is within one of a first rangewithin the cardiac event zone corresponding to a ventricular tachycardiaevent and a second range within the cardiac event zone corresponding toa ventricular fibrillation event.
 6. The device of claim 1, wherein theO₂ variation index trend corresponds to relationships betweenproportions of red light intensities and infrared light intensities tocorresponding baseline intensities.
 7. The device of claim 1, whereinthe O₂ variation index trend corresponds to$\frac{i}{i_{0}} - \frac{i^{*}}{i_{0}^{*}}$ wherein is an intensity ofred light, i* is an intensity of infrared light, i₀ is an intensity ofred light baseline, and i₀* is an intensity of infrared light baseline.8. A method of determining a cardiac event in a medical device,comprising: sensing cardiac signals from a plurality of electrodes;detecting the cardiac event in response to the sensed cardiac signals;sensing physiologic signals and acquiring a plurality of O₂ variationindex samples corresponding to the sensed physiologic signals; computingan O₂ variation index trend associated with a predetermined number of O₂variation index samples of the plurality of O₂ variation index samplesin response to detecting the cardiac event; determining, in response tothe computed O₂ variation index trend being less than a variation indextrend threshold, whether a rate of change of the computed O₂ variationindex trend is within a predetermined cardiac event zone defined by afirst zone threshold having a first rate of change corresponding to theO₂ variation index trend becoming less than the variation index trendthreshold in a first period of time, and a second zone threshold havinga second rate of change corresponding to the O₂ variation index trendbecoming less than the variation index trend threshold in a secondperiod of time greater than the first period of time; and confirming thedetermined detected cardiac event in response to both the computed O₂variation index trend being less than the variation index trendthreshold and the rate of change of the O₂ variation index trend beingwithin the predetermined cardiac event zone.
 9. The method of claim 8,wherein the O₂ variation index trend corresponds to a measure of changein tissue oxygenation and includes a start point corresponding to afirst O₂ variation index sample of the predetermined number of O₂variation index samples and an endpoint corresponding to a second O₂variation index sample of the predetermined number of O₂ variation indexsamples.
 10. The method of claim 8, further comprising determining adeviation of the predetermined number of O₂ variation index samples fromthe O₂ variation index trend.
 11. The method of claim 10, furthercomprising determining, in response to the determined deviation, whetherthe sensed cardiac signals are associated with an unstable rhythm,wherein the determined cardiac event is confirmed in response to thedetermined deviation and the sensed cardiac signals being associatedwith an unstable rhythm.
 12. The method of claim 8, wherein the O₂variation index trend corresponds to relationships between proportionsof red light intensities and infrared light intensities to correspondingbaseline intensities.
 13. The method of claim 8, wherein the O₂variation index trend corresponds to$\frac{i}{i_{0}} - \frac{i^{*}}{i_{0}^{*}}$ wherein i is an intensity ofred light, i* is an intensity of infrared light, i₀ is an intensity ofred light baseline, and i₀* is an intensity of infrared light baseline.14. The method of claim 8, further comprising determining whether therate of change is greater than the first zone threshold and less thanthe second zone threshold.
 15. The method of claim 8, further comprisingdetermining whether the rate of change is within one of a first rangewithin the cardiac event zone corresponding to a ventricular tachycardiaevent and a second range within the cardiac event zone corresponding toa ventricular fibrillation event.
 16. A method of verifying a cardiacevent in a medical device, comprising: sensing physiologic signals andacquiring a plurality of O₂ variation index samples corresponding to thesensed physiological signals; computing an O₂ variation index trendassociated with a predetermined number of O₂ variation index samples ofthe plurality of O₂ variation index samples; determining whether a rateof change of the O₂ variation index trend is within a predeterminedcardiac event zone defined by a first zone threshold corresponding to arate of change of O₂ variation index samples associated with anon-physiologic event and a second zone threshold associated with a rateof change of O₂ variation index samples associated with a sinus rhythmevent; and verifying the cardiac event in response to the determined O₂variation index trend and the determining whether a rate of change iswithin a predetermined cardiac event zone.
 17. The method of claim 16,wherein the O₂ variation index trend corresponds to a measure of changein tissue oxygenation and includes a start point corresponding to afirst O₂ variation index sample of the predetermined number of O₂variation index samples and an endpoint corresponding to a second O₂variation index sample of the predetermined number of O₂ variation indexsamples.
 18. The method of claim 16, further comprising determiningwhether the computed O₂ variation index trend is less than apredetermined index trend threshold.
 19. The method of claim 18, furthercomprising determining whether a deviation of the predetermined numberof O₂ variation index samples from the computed O₂ variation index trendis greater than a deviation threshold, wherein the cardiac event isverified in response to both the computed O₂ variation index trend beingless than the predetermined index trend threshold and the deviation notbeing greater than the deviation threshold.
 20. The method of claim 19,wherein the O₂ variation index trend corresponds to relationshipsbetween proportions of red light intensities and infrared lightintensities to corresponding baseline intensities.
 21. The method ofclaim 16, wherein the O₂ variation index trend corresponds to$\frac{i}{i_{0}} - \frac{i^{*}}{i_{0}^{*}}$ wherein i is an intensity ofred light, i* is an intensity of infrared light, i₀ is an intensity ofred light baseline, and i₀* is an intensity of infrared light baseline.22. The method of claim 16, further comprising determining whether therate of change is one of greater than first zone threshold and less thansecond zone threshold.
 23. The method of claim 16, further comprisingdetermining whether the rate of change is within one of a first rangewithin the cardiac event zone corresponding to a ventricular tachycardiaevent and a second range within the cardiac event zone corresponding toa ventricular fibrillation event.